On the effective equation to describe the spreading of a small viscous drop over a rough surface

Many industrial and material processing operations require the spreading of a liq­ uid on a solid. For example, removal of dirt by detergent solutions, drop-wise condensation of liquids, and displacement of one liquid by another in soils. Fre­ quently, the liquid may be a paint, a lubricant, an ink, or a dye. The solid may ei­ ther show a simple surface, or be finely divided, as in the case of suspensions, po­ rous media, or fibers.